1. Field of the Invention
The present invention relates to a phase shifting circuit, and more particularly, to a phase shifting circuit necessary for a quadrature detection of an FM signal.
2. Description of the Prior Art
In a conventional quadrature detector used in a frequency modulation (FM) radio receiver, as shown in FIG. 1, a received FM radio signal is transmitted through two lines 101a and 101b from an intermediate frequency (IF) circuit 100 which converts the frequency of the FM radio signal into an FM intermediate frequency. Through the line 101a, the signal is transmitted directly to a multiplier 103, whereas through the line 101b, the signal is transmitted to the multiplier 103 after its phase is shifted by 90.degree. by a phase shifting circuit 102. These two signals are multiplied at a multiplier 103. The phase shifting circuit 102 shifts the phase by a capacitor C and a coil L.
In a conventional detector shown in FIG. 2, a ceramic resonator 104 is used in a phase shifting circuit 105. In the phase shifting circuit 105 of this method, as shown in FIG. 3, three resistors R and the ceramic resonator 104 form a Wheatstone bridge, and the phase of the FM signal is shifted by a reactance component of the ceramic resonator 104.
However, in the former phase shifting circuit, it is necessary to regulate the frequency by the capacitor or the coil, and such a regulation, which is a mechanical regulation, is liable to deterioration with age. In addition, since the coil cannot be formed in an integrated circuit (IC), the number of necessary externally-attached terminal pins of the IC increases, and an externally-attached part (i.e. coil) is required. In the latter phase shifting circuit, not only an externally-attached part is required but also the part (i.e. ceramic resonator) is not available in some countries since it is a special part. In addition, the ceramic resonator is inferior to the former method in characteristics, and is difficult to match with an IC.